Various forms of plastic identification cards bearing a stripe of magnetic medium have come into widespread use. Particularly, magnetic stripe cards have been used to access various systems ranging from automated teller machines to credit verification machines. Equipped with magnetic transducers for sensing magnetic stripes, these systems readily sense the magnetic stripe cards for retrieving and storing data.
Like ferro-magnetic materials in general, the core of a magnetic head may exhibit hysteresis when exposed to high field strengths and retain a net remanent magnetization, even when the magnetizing field intensity is zero. Consequently, a coercive field is required to bring the magnetization back to zero.
Various magnetic cards are in widespread use, some being more desirable than others. Specifically, magnetic cards requiring greater magnetizing fields are highly desirable in that they afford better resistance to demagnetization. The magnetic cards in this category are known as "high-coercivity" cards, in contrast to "low-coercivity" cards, and the former provides for magnetic stripes having 600 or more oersteds.
Typically, modern magnetic stripe card transducers (heads) are designed for processing high-coercivity cards and retain a significant residual magnetism. Such residual magnetism is known to damage the records on low-coercivity cards during subsequent processing.
The need to degauss magnetic transducers or magnetic heads has been recognized in the past. Various past techniques include exposing the heads to alternating magnetic field of decreasing intensity or directing an exponentially decaying alternating current signal through the windings of the head itself. However, such techniques are not conventionally used to degauss magnetic transducers for sensing magnetic stripe cards. With respect to magnetic stripe cards, it is desirable to accomplish degaussing with low cost in a digital system. Thus, a need exists for a system incorporating a magnetic transducer degausser, which system degausses with little or no additional equipment and avoids damage to the recordings on low-coercivity magnetic stripe cards.
The present invention is based on recognizing a desirable magnetic transducer for reading a magnetic-stripe card of either high or low coercivity without erasing, or otherwise damaging, information stored on low-coercivity magnetic stripes. Specifically, the present invention incorporates a degaussing unit which for example, exposes a magnetic transducer to an electrical pulse, or a gradually increasing drive frequency, to cancel the residual magnetism of the transducer.